1. Field of the Invention
The present invention pertains to a process for making block copolymers containing a reactive functional group, such as anhydride, epoxy, amine, amide, hydroxyl or acid groups, in two or more blocks via free radical polymerization in the presence of a stable free radical, a composition of matter comprising block copolymers containing a reactive monomer or monomers in two or more blocks via free radical polymerization and use of the composition of matter as a compatibilizer in blending polymers.
2. Description of the Related Art
The blending of polymers provides a powerful route for obtaining materials with improved property/cost performances. Since most polymer pairs are immiscible, a compatibilization strategy is required to obtain maximum synergy of properties. This strategy is usually cheaper and less time-consuming than the development of new monomers and/or new polymerization routes, as the basis for entirely new polymeric materials. An additional advantage of polymer blends is that a wide range of material properties is within reach by merely changing the blend composition. Compatibilization of polymer blends can be achieved using compatibilizers, which are macromolecular species exhibiting interfacial activities in heterogeneous polymer blends. Usually the chains of a compatibilizer have a blocky structure, with one constitutive block miscible with one blend component and a second block miscible with the other blend component. Another option for compatibilization is the addition of a reactive polymer, miscible with one blend component and reactive towards functional groups attached to the second blend component, which results in an “in-situ” formation of block or grafted copolymers. This technique has certain advantages over the addition of pre-made block or grafted copolymers. Usually reactive polymers can be generated by free radical copolymerization or by melt grafting of reactive groups on to chemically inert polymer chains. Furthermore, reactive polymers only generate block or grafted copolymers at the site where they are needed, i.e. at the interface of an immiscible polymer blend.
The successful development of compatibilizers that permit composites of polyolefins such as polypropylene and minerals, glass and/or polar thermoplastics, having excellent physical properties was rapid. By the early 1970's compatibilizers based on maleated polypropylene were available for the manufacture of polyolefin-based composite materials. The maleic anhydride moieties of these compatibilizers is reacted with the nucleophilic amines and hydroxyl functional groups in polyamides, polyesters and polycarbonates and with the amino silanes used to modify the surface of glass fibers and other mineral fillers.
Attempts to apply the analogous solution to the other major hydrocarbon polymer group, styrenics, have been without success. Maleation of polystyrene is random along the polystyrene chain and is not located on the ends of the chain, as in the case for polypropylene. Similarly, copolymerization of styrene monomer and maleic anhydride yields an alternating copolymer, and copolymerization of styrene with other nucleophile reactive monomers is random along the polystyrene chain. Such candidate compatibilizers contain functional groups that are reactive with the nucleophiles present in the polar thermoplastics and amine modified fillers and therefore interact with the polar phase of the composites (e.g., glass, minerals, and or polar thermoplastic polymers), yielding in some cases more uniform dispersions of the one material in the other. However, because the architecture of these candidate compatibilizers is random, there are no separate domains, and therefore, no domain that is compatible with the styrene phase of the composite and sufficiently long to chain entangle with the polystyrene in the composite. As a result, even with improved dispersion of one phase in the other, the required improvement in the physical properties of the alloy material is not achieved, and, indeed, sometimes there is even a degradation of physical properties compared to the same alloy without the candidate compatibilizer (Dong, C., et. al., Polymer, 1996, 37, 14, 3055-3063; Chang, F., et al., Polym. Eng. Sci., 1991, 31, 21, 1509-1519; Jannasch, P., et. al., J. Appl. Polym. Sci., 1995, 58, 753-770).
The successful strategy with polyolefin composites and failure in polystyrene composites was studied and reported by Fumio Ide (Ide, F., et. al., J. Appl. Polym. Sci., 1974, 18, 4, 963-74). As mentioned in U.S. Pub. No. 2005/004310 A1, researchers recognized that the presence of reactive functional groups like maleic anhydride were necessary in compatibilizers, but not sufficient for good compatibilization. In addition to this, the placement of the nucleophile reactive functional groups within the compatibilizer polymer architecture has been random. Compatibilizer materials that present a block copolymer structure, in which each one of the blocks is thermodynamically compatible with one of two polymeric materials to be blended, perform more effectively as compatibilizers than their random copolymer counterparts (U.S. Pub. No. 2004/0077788A1). Well-defined styrene block copolymers containing reactive groups have been prepared and applied as reactive compatibilizers, but they usually exhibit important disadvantages, such as: i) complex synthetic techniques, ii) the presence of unstable and corrosive moieties and iii) the addition of an extraneous polymer with different chemical and physical properties (Park, C., et. al., Polymer, 2001, 42, 7465-7475; U.S. Pat. No. 6,417,274 B1; Koulouri, E. G., et. al., Macromolecules, 1999, 32, 6242-6248).
In order to obtain well defined block copolymers to be used as compatibilizers, several approaches have been taken, and one approach is the use of living polymerization processes. Living polymerization processes, in which termination reactions are suppressed or significantly reduced, allow for the formation of block copolymers, as the life of each individual chain is extended to periods comparable to the duration of the process (minutes or hours). It is possible to produce block copolymers with functional groups by anionic polymerization, but this technique presents severe limitations for its broad practical application. On one hand, it requires conditions of extreme purity in the monomers because humidity traces destroy the catalyst, and for many monomers it is very difficult to control, requiring extremely low temperatures. Also, the polymerization of monomers having functional groups is not practical since the catalyst can be destroyed by the presence of a number of functional groups. As a result, the industrial application of this technique is reduced to a few monomers, leaving out technologically-important functional monomers.
Due to limitations in the anionic polymerization process, a more promising technique for producing block copolymers with a large variety of monomers is that based on living or quasi-living free radical polymerization. This can be achieved by adding, to an otherwise standard free radical polymerization recipe, a chemical agent that significantly reduces the extent of irreversible termination or chain transfer reactions, conferring a living or quasi-living character to the polymerization, which is also called “controlled polymerization” or “controlled free radical polymerization.” There are several ways to obtain this behavior (Sawamoto, et. al., Chem. Rev. 2001, 101, 3689-3745), but most of them are limited in an industrial practice because they require chemical agents that are not readily commercially-available in the market. Among these techniques, one that is particularly useful, and for which the required chemical agents are available in the market, is a quasi-living free radical polymerization controlled by nitroxides (nitroxide mediated radical polymerization, NMRP), and derivatives thereof (like alcoxyamines, U.S. Pat. No. 6,455,706 B2, which act as stable free radicals capping polymeric growing radicals and uncapping them in a fast and reversible way, allowing for short periods of propagation through monomer-addition steps (U.S. Pat. No. 5,401,804; EP 0 869 137 A1; U.S. Pat. Nos. 6,258,911 B1; 6,262,206 and 6,255,448 B1).
Nitroxide mediated radical polymerization or NMRP has been used to prepare diblock copolymers as additives for preparing lubricating oil compositions as reported by Visger et. al. (U.S. Pat. No. 6,531,547 B1), and recently, it has been used as a technique to obtain pure diblock copolymers that are able to act as compatibilizers in polymer blends. U.S. Patent Application Pub. No. 2005/0004310, filed by Hong et al., discloses the compatibilization of a styrenic polymer/polyamides or styrenic polymer/glass, using diblocks of styrene and a styrenic reactive block. The reported technique involves the purification of the first synthesized block (diluting with THF, adding methanol or water/methanol and drying) before adding the second monomer in order to have a pure polystyrene block. A variation of this approach that has been successfully applied in polyphenylene ether-polyamide blends (U.S. Pat. No. 6,765,062 B2) is the synthesis of end-functionalized polymers using a functional alcoxyamine (U.S. Pat. No. 6,566,468 B1; U.S. Pub. No. 2004/0049043A1). This approach requires a special controlling agent bearing epoxy functionality, which is not believed to be commercially available, and is more expensive than simple controlling agents such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or TEMPO derivatives. Another approach to the synthesis of pure block copolymers is taking advantage of the natural alternate polymerization of certain monomers like maleic anhydride and styrene in the presence of a nitroxide in order to control molecular weight and polydispersity, as described in the present assignee's U.S. Patent Application Pub. No. 2004/0077788A1, entitled Block Copolymers Containing Functional Groups, which is incorporated by reference.
Successful reactive compatibilizers described in the prior art are nonrandom block polymers, based on copolymers consisting of one reactive block and one nonreactive block or in special cases, only one reactive monomer at the polymeric chain end. However, in order to obtain pure blocks, an intermediate purification step is required, such as solvent evaporation, precipitation and evaporation, and the purification step increases the cost of the process. Only in the case where the monomers naturally create an alternating composition, such as in the case of styrene and maleic anhydride, are blocks formed as a consequence of reactivity, where a purification step is not required. Consequently, there remains a need for improvements in the field of compatibilizers.